The invention relates to a digital wireless communication system. In particular, the invention relates to cordless telephones and cellular networks.
A common type of wireless communication is the communication which occurs between a cordless telephone handset and its associated base unit within a limited distance range around the user""s home or business. Cordless telephones typically operate over a radio frequency (RF) portion of the spectrum set aside for general public use. Also, the power of the cordless phone signal is lower than other communication signals, because the signal needs to only be transmitted between the cordless handset and the associated base unit within the home or business of the user. Therefore, there is no user license from the Federal Communications Commission (FCC) required to operate a cordless telephone. Ultimately the communication is carried from the associated base unit along a landline on the public telephone network to the connecting party; therefore the cost of the telephone call is regulated by the LEC which owns the public switched telephone network (PSTN).
Another method of wireless communication is a regional cellular communication network which is operated by a cellular operator to enable the transmission of voice and data from a mobile station to a cellular base station over a specific band of frequencies, e.g., 824-849 MHz and 869-894 MHz, under license by the FCC. The bands are generally broken up into transmission channels and reception channels which each employ different bands of frequencies in the cellular spectrum. Cellular transmissions from the base station to the mobile station occupies the spectrum between 824 and 849 MHz with each transmission channel occupying about 30 KHz Cellular reception from the mobile stations to the base stations generally occupy the spectrum between 869 and 894 MHz with each reception channel occupying about 30 KHz. As is well known to those of ordinary skill in the art, each of the transmission and reception bands are divided between two cellular service providers in each market and are referred to as xe2x80x9cAxe2x80x9d and xe2x80x9cBxe2x80x9d bands. Thus, each provider operates four hundred and sixteen pairs of transmission and reception frequency channels on which to provide service. Twenty-one of the four hundred and sixteen frequency channels pairs are ordinarily used as control channels to send control signals from the base station to the mobile station, thus only three hundred and ninety-five channels are actually available to transmit calls between the cellular base station and mobile station. The cellular service provider enlists subscribers who are authorized to communicate via the regional cellular network. Each subscriber must purchase a mobile station or handset which is capable of communication with the regional cellular network. The handset, at the time of manufacture, is assigned an electronic serial number (ESN). The ESN is generally stored in the permanent memory, such as an EEPROM, in the handset. The subscriber registers the mobile station with the regional cellular network and the mobile station is assigned a mobile identification number (MIN) by which the mobile station can be accessed. As the price of mobile stations decreases and the cost of air time decreases, the number of users that subscribe to regional cellular networks is increasing.
Recently, handsets have been disclosed which are capable of switching between communication with a cellular network and with an RF cordless telephone until Once the handset is in the proximity of the cordless telephone unit, the telephone unit is capable of receiving calls from both the public switched telephone network (via the RF cordless telephone unit) and the regional cellular network. Since the cellular and RF cordless communication systems utilize different frequency bands for communication, a handset that can communicate with both cellular and RF cordless base stations requires some significant additional hardware and software. In practice, a handset that can communicate with both types of base stations requires one transceiver that can communicate with cordless frequencies and one transceiver that can communicate with cellular frequencies as well as separate interface hardware between each transceiver and the main handset controlling hardware. Further, the main handset controlling hardware must be able to recognize and communicate with the different communications protocols required to communicate with the RF cordless telephone base station and with the cellular networks. The additional hardware required to communicate with both base stations increases the size and the weight of the handset. Further, the additional hardware increases the cost of the handset in a highly price-competitive market.
The preferred embodiment of the present invention comprises a cordless cellular base system. A cordless cellular base station is capable of communicating with a cellular network compatible mobile unit, also referred to as a mobile station herein. The cordless cellular base station, also referred to as a subregional basestation, is preferably connected to a landline on a public switched telephone network and is assigned a landline number or phone number. The mobile station is registered with a cellular network and is assigned a mobile identification number. Advantageously, the mobile station is capable of communication with both a conventional regional cellular base station and to the cordless cellular base station utilizing the same cellular frequency range and communications protocol. When the mobile station is communicating with the cellular network, it is referred to as being in the regional cellular service mode. When the mobile station is communicating with the cordless cellular base station, it is referred to as being in cordless cellular telephone landline service mode.
The mobile station of the present invention advantageously communicates with the cordless cellular base station and with the regional cellular base stations of the cellular network utilizing the same frequency range and the same communications protocol. In a preferred embodiment, the communications protocol that is utilized is compliant with the IS-136, parts one and two, air interface standard which is hereby incorporated by reference in its entirety. The IS-136, parts one and two, standard is available from the Telecommunications Industry Association (TIA), Engineering Dept., 2001 Pennsylvania Avenue, N.W., Washington, D.C. 20006. This standard provides in part for TDMA (time division multiple access) digital communications and is well known to those of skill in the art The IS-136 standard is designed to ensure compatibility between cellular mobile telecommunication systems so that a mobile station can obtain service in any cellular system manufactured in accordance with the standard. Since the mobile station communicates with the cordless cellular base station and with the regional cellular base stations of the cellular network utilizing the same frequency range and the same communications protocol, the mobile station may be manufactured using similar hardware, such as a digital transceiver, and similar software to communicate with both the cordless cellular base station and the cellular network. Therefore, the overall size and the weight of the mobile station is not increased compared to conventional cellular telephones which are compatible with the IS-136 standard. Further, by reducing the amount of additional hardware and additional software necessary, the cost of the mobile station of the preferred embodiment is not much higher than conventional IS-136 compliant mobile stations. The cordless cellular base station is able to communicate with the mobile station and act as a conduit between the mobile station and the public switched telephone network. Further, despite the fact the mobile station is IS-136 compliant and can communicate with both analog and digital regional cells, the mobile station preferably communicates with the cordless cellular base station utilizing a digital control channel and corresponding digital traffic channels. By utilizing a digital channels, rather than analog and digital channels for communication with the mobile station, the hardware and software required to operate the cordless cellular base station is further reduced and thus the manufacturing costs are reduced.
In one aspect of the present invention, in order to maintain a secure system, the cordless cellular base station must first register with the cellular network before its operation is enabled. In a preferred embodiment, the cordless cellular base station is not operational without the network authorization, as the cellular network provides the cordless cellular base station with certain operating parameters, such as a list of authorized frequencies for its operation. After the network authorization procedure is complete, the cordless cellular base station enables registration of mobile stations. In the preferred embodiment, once a mobile station receives registration privileges with a particular cordless cellular base station, the mobile station automatically registers with the cordless cellular base station when the mobile station comes into proximity with the cordless cellular base station. As the number of cordless cellular base station users increases it becomes more likely that at least some cordless cellular base stations will be operating in close proximity to one another. With base stations present in adjoining houses, for example, it is not desirable to enable automatic registration for all mobile station users that come into proximity with a cordless cellular base station, because it is possible for one neighbor to inadvertently automatically register with another neighbor""s cellular base station. Advantageously, the automatic registration feature of the present invention allows the cordless cellular base station to restrict automatic registration to those users who have been previously pre-registered with a particular cordless cellular base station. By requiring preregistration, before automatic registration occurs, accidental automatic registration with a nearby cordless cellular base station is prevented while still providing the convenience of automatic registration for frequent users. In addition, the mobile station will not attempt to automatically register with a cordless cellular base station unless it has previously registered with that cordless cellular base station and knows on which channels to look for the cordless cellular base station.
In another aspect of the present invention, the cordless cellular base station maintains a cordless cellular base station registration list in a semipermanent portion of memory which stores the mobile system identification number of the mobile stations which have been previously been granted registration privileges with the cordless cellular base station. For each entry, the cordless cellular base station registration list stores a status record that indicates the state of the mobile station with respect to the cordless cellular base station and the mobile station identification number. Preferably, there are three states of a mobile station with respect to the cordless cellular base station which are referred to as the: xe2x80x9cstandbyxe2x80x9d, xe2x80x9cactivexe2x80x9d and xe2x80x9cdormantxe2x80x9d states. The dormant state indicates that the mobile station which has previously registered with the cordless cellular base station is not currently registered with the cordless cellular base station. The standby state indicates that the mobile station is currently registered with the cordless cellular base station and is not currently on a call, i.e., the mobile station is xe2x80x9cstanding byxe2x80x9d waiting to receive or place a call through the cordless cellular base station. The active state indicates that the mobile station is registered with the cordless cellular base station and is currently on a call which is being controlled by the cordless cellular base station. In a preferred embodiment the status of up to ten mobile stations can be monitored by the cordless cellular base station.
In another aspect of the present invention, when a mobile station which is not presently involved in a call comes within range of a cordless cellular base station with which it has previously registered, in accordance with the preferred embodiment, it automatically switches from regional cellular service mode to cordless telephone landline service mode without user intervention. Once the mobile station has confirmed with the cordless cellular base station that the mobile station has switched to cordless landline service mode, the cordless cellular base station communicates with the cellular network to provide a location update message. If the mobile station has a call in progress, the mobile station waits for the call to be completed before it is switched to cordless telephone landline service mode and sends the call forwarding update message.
Once the cellular network receives the call forwarding update, the cellular network routes all calls to the mobile station""s mobile identification number to the landline number associated with the cordless cellular base station. In addition, while the mobile station remains within range of the cordless cellular base station, all calls placed from the mobile station are sent through the cordless cellular base station to the associated landline. In the preferred embodiment there is no hand-off of telephone calls between the regional cellular network and cordless cellular base station environments. For example, if a call is initiated via the cordless cellular base station and the mobile station is moved to an area outside of the cordless cellular base stations range, the call is disconnected instead of being transferred to the cellular network. In an alternate embodiment, handoff of telephone calls between the regional cellular network and cordless cellular base station environments is enabled. For example, if a call is initiated via the cordless cellular base station and the mobile station moves to an area outside of the cordless cellular base station""s range, the call is automatically transferred from the cordless cellular base station and its associated landline to the cellular network. The cellular network then routes the call through the cellular base station and a hand-off is performed to the user""s mobile station as is performed in a conventional cellular system.
By communicating with the cellular network, the cordless cellular base station is advantageously able to inform the cellular network where to route telephone calls for the mobile station""s identification number when the mobile station is registered with the cordless cellular base station ensuring that the mobile station user will always receive telephone calls for its mobile station identification number regardless of the mode of operation of the mobile station.
Typically, when the mobile station severs contact with the cordless cellular base station, the cordless cellular base station sends a network forwarding cancellation message to the cellular network to cancel the forwarding of calls for the mobile station identification number to the landline number associated with the cordless cellular base station. The cordless cellular base station is informed during an initial authorization message which types of registration/deregistration activities for which the network wants to be informed. Some examples of types of deregistration events for which the network may want to be contacted are: when the mobile station is turned off, i.e., a power down deregistration or due to a manual cancellation of the cordless service mode, i.e., forced deregistration. When the mobile station severs contact with the cordless cellular base station for reasons other than powering off, the mobile station typically registers with the local regional cellular base station of the regional cellular network. Once the mobile station is registered with the regional cell, calls to the users mobile identification number are directly routed by the cellular network to the mobile station.
In another aspect of the present invention, the cordless cellular base station can process up to two active mobile stations at the same time. When two mobile stations are listed as active, the cordless cellular base station bridges the audio signal for the two mobile stations together such that each of the mobile stations is acting like an extension on a normal landline. In a first embodiment, each mobile station receives a voice signal which is a summation of the voice signals from the caller on the landline and from the other mobile station. In a second embodiment, the cordless cellular base station compares a voice signal from one mobile station with the voice signal from the caller on the land line and whichever is loudest is sent to the other mobile station. The voice signals from each of the mobile stations are summed together and sent to the landline. Thus, the landline caller hears a summation of both mobile stations speaking while each mobile station only hears whichever signal is louder, the other mobile station or the caller on the landline. In a third embodiment, the cordless cellular base station uses a detector to determine which of the signals from the mobile stations is the loudest and sends the loudest received signal to the caller on the landline. The users of the mobile stations are able to listen to the voice of the party on the landline; however, they are unable to hear what is being said by the user of the other mobile station.
The cordless cellular base station is designed to operate in the residential home or small office environment. This active RF environment can be potentially very noisy because there may be no dedicated frequency spectrum allocated for the cordless cellular base station operation coupled with the fact that the frequency usage is not explicitly coordinated with the regional cellular network planning. The cordless cellular base station has to co-exist in the same cellular band used by the regional cellular network and views the regional cellular network as a source of background interference. The cordless cellular base station attempts to avoid the potential interference by the cellular network by choosing frequencies which, as far as the cordless cellular base station can determine, are not being used by nearby regional cells or by other nearby cordless cellular base stations.
In another aspect of the present invention, the cordless cellular base station implements an avoidance mechanism which scans the frequencies in the cellular band and determines the best and next-best cellular frequencies for communication with the cellular network at all times. The cordless cellular base station periodically takes received signal strength (RSS) measurements for each of the authorized frequencies of the cordless cellular base station and word error rate (WER) measurements on the current operational frequency when a call is in progress, which are measurements known to those of skill in the art. The cordless cellular base station translates the RSS measurements into a score increment or decrement value based upon a stored score increment table. After each measurement, the current score increment/decrement value adjusts the previous score value. The score for a frequency is a measure of the amount of noise plus interference at a given frequency, thus the score itself is a measure of the potential of interference if this channel were selected rather than the actual interference occurring on the channel. Therefore, throughout this document any discussion of the measurement of the interference on a channel should be interpreted as the measurement of the potential interference on this channel, if the channel was selected for operation. Thus, the interference score represents the potential interference that would be encountered on that frequency, with a higher score representing more interference, and a lower score representing lower interference. In the absence of interference, the score value will gradually decay towards zero as more measurements are made.
The cordless cellular base station uses the interference score measurements in making the choice of an operating frequency. By selecting a frequency from those with the lowest interference scores, and by using appropriate channel abandonment thresholds described below, the cordless cellular base station attempts to avoid transmitting on any frequency which is already in use nearby the public or private cellular network or by other cordless cellular base stations within range. Preferably, the cordless cellular base station selects for its initial operational frequency, the frequency with the lowest interference score. The cordless cellular base station 10 randomly selects for its backup frequencies, a specified number of downlink frequencies whose scores are below a high threshold value (Ht). Depending upon whether a call is in progress or whether a primary mobile station is registered, if the interference score of the current operational frequency rises above a first low threshold (Lt) or above the high threshold (Ht), the cordless cellular base station 10 automatically switches its operational frequency to the first backup frequency as described in more detail below. The cordless cellular base station 10 also removes backup frequencies from the back-up frequency list if the interference score for that backup frequency rises above the high threshold (Ht).
In another aspect of the present invention, the cordless cellular base station needs to locate a specified number of frequencies having score values below the high threshold value (Ht). If a sufficient number of frequencies are not available, the cordless cellular base station notifies the cellular network of the problem. In one embodiment, when the cordless cellular base station notifies the cellular network of the problem, the cellular network will provide the cordless cellular base station with a list of alternative authorized frequencies. In another embodiment, the cellular network will temporarily lower the specified number of frequencies having score values below the high threshold value (Ht) that are needed, thus enabling the cordless cellular base station to continue to operate. In still another embodiment, if an insufficient number of frequencies are available, the cordless cellular base station may continue to take frequency measurements and will cease transmission until a sufficient number of clear frequencies are available.
The cellular network initially provides the cordless cellular base station in an authorization message a list of operational frequencies in the cellular spectrum in which to operate. In one embodiment, the list includes all of the possible voice channels in the cellular spectrum. In another embodiment, the list includes a small portion of the entire cellular spectrum.
In another aspect of the present invention, the cordless cellular base station periodically initiates a phone call to a cordless cellular base station visitor location register (CCBS VLR). The CCBS VLR is a data base which is used to keep track of the location of visiting mobile stations which have registered with a cordless cellular base station. In one embodiment, the CCBS VLR contains additional storage space to receive data regarding interference scores for the cellular frequencies which are reported by the cordless cellular base stations. After receiving the initial call from the cordless cellular base station, the cellular network sends a message to the cordless cellular base station indicating that it is ready to receive the interference score data. Next, the cordless cellular base station downloads all of the interference scores for all of its authorized frequencies to the CCBS VLR. The CCBS VLR forwards the interference score information to a data collection node in the cellular network. The cellular network uses these interference scores to assist in assigning or re-assigning the operational frequencies for the regional cells.
In another aspect of the present invention, during any contact between the cellular network and the cordless cellular base station, such as during a location update or a network cancellation procedure, the cellular network has the ability to update the operational parameters in the cordless cellular base station by sending the cordless cellular base station a new authorization message. The new authorization message may be sent in response to any request sent to the cellular network. The update is useful for revising the cordless cellular base station operation to accommodate for changes in the service in the area around the cordless cellular base station and to update the cordless cellular base station regarding changed features of the cellular network. For example, the cellular network may temporarily alter the telephone number(s) that the cordless cellular base station calls to access the location update/call forwarding feature or the cellular network may revise the allowable frequencies of the cordless cellular base station.
In another aspect of the present invention, in order to enable the cellular network to periodically change the operational parameters of the cordless cellular base station, the cordless cellular base station preferably includes a preset timer which counts down the amount of time since the cordless cellular base station last contacted the cellular network. When the timer expires, the cordless cellular base station automatically calls the cellular network. In the preferred embodiment, the timer is set for thirty days, thus if the cordless cellular base station has not contacted the cellular network within the last thirty days, for example, to request a location update for a registered mobile station, the cordless cellular base station automatically contacts the cellular network. In response, the cellular network determines if it is necessary to send the cordless cellular base station a new authorization message. If a new authorization message is required, the cellular network sends the message to the cordless cellular base station. If a new authorization message is not required, the cellular network sends a return result message to the cordless cellular base station. Upon receiving either message from the cellular network, the timer in the cordless cellular base station is reset for thirty days. This feature is useful in preventing fraudulent usage of a cordless cellular base station. For example, if the owner did not pay his/her bill for the service and also stopped using his or her phone away from the cordless cellular base station, it would be difficult for the cellular network to cancel the operation of the cordless cellular base station, because the cordless cellular base station would never contact the cellular network With the automatic contact feature, the cordless cellular base station would automatically contact the cellular network within the time specified on the preset timer. The network would, if necessary, send a new authorization message removing all of the operating frequencies from the cordless cellular base station which belonged to the non-paying customer, thus making the cordless cellular base station inoperable.
In another aspect of the present invention, when a call is initiated by a mobile station that is registered with the cordless cellular base station, the cordless cellular base station initially checks to see if the phone number matches the MIN for one of the other mobile stations which is listed in standby mode in the registration table on the cordless cellular base station. If the phone number matches a MIN for the one of the mobile stations listed in standby mode, the cordless cellular base station pages that mobile station and initiates an intercom conversation between the two mobile stations. The intercom feature is described in more detail below. The intercom conversation does not utilize the landline, so the telephone extensions connected directly to the landline can initiate and receive calls over the landline.
In another aspect of the present invention, when a user wants to use a mobile station which was not previously registered with the cordless cellular base station, the user presses the initial registration button on the cordless cellular base station which initiates an initial registration procedure. The user of the mobile station may then depress a test registration function key on the mobile station which causes the mobile station to send out a test registration message to the cordless cellular base station. The test registration message is specified by the IS-136 standard. The cordless cellular base station checks to see if the test registration message is received from a mobile station within a specified time period, preferably thirty seconds. If a test registration message is received, the cordless cellular base station determines if the registration list is full. If there is no room in the registration list for an additional mobile station, the cordless cellular base station bumps off a dormant non-primary mobile station from the list If there are no non-primary mobile stations which are dormant, the cordless cellular base station sends a negative test registration response to the mobile station and the initial registration attempt is terminated. If there is room in the registration list for an additional mobile station, the cordless cellular base station sends a positive test registration response message to the mobile station. Upon receiving a positive test registration response, the mobile station will display a set of alpha characters transmitted by the cordless cellular base station in the positive test registration response message which are then stored by the mobile station. In the preferred embodiment, the alpha characters which are displayed form the word xe2x80x9cCordlessxe2x80x9d. At this time, the mobile station may decide to attempt cordless registration with the cordless cellular base station. If the user wishes to attempt the registration, the mobile station sends a registration message to the cordless cellular base station. If a registration is not desired, no message is sent to the cordless cellular base station. Once the cordless cellular base station receives the registration message, the cordless cellular base station completes the registration procedure and sends a registration accept message to the mobile station. In subsequent automatic registration attempts, once the mobile station recognizes the Residential System ID (RSID) of the cordless cellular base station, the mobile station will display the stored alpha characters, such as, xe2x80x9cCordlessxe2x80x9d to indicate that the mobile station has automatically registered with the cordless cellular base station.
In another aspect of the present invention, the cordless cellular base station advantageously utilizes two separate antennasxe2x80x94a transmit (Tx) antenna and receive (Rx) antenna Preferably, the transmit and receive antennas are simple antennas, since the coverage area for the cordless cellular base station is small. In a preferred embodiment, the transmit antenna and receive antenna are physically separated on the cordless cellular base station. Further, their signals are transmitted to the cellular transceiver along separate paths to eliminate the need for a duplexer, thereby simplifying the design and reducing manufacturing costs.
Advantageously, in another aspect of the present invention, a single receiver is used by the cordless cellular base station to both receive the mobile station transmissions and to take noise measurements which saves in the cost of the cordless cellular base station and reduces the size of the cordless cellular base station. In order to accurately receive the signals from the mobile station, the sensitivity of the receiver on the cordless cellular base station can be significantly less than the sensitivity to measure the noise in the surrounding environment. Advantageously, when the receiver is waiting to receive a mobile station transmission, the sensitivity of the receiver is lowered to enable it to receive higher powered transmission. When the receiver is taking an interference measurement, the sensitivity of the receiver is increased to enable it to receive lower power transmissions. By changing the sensitivity of the receiver for normal reception and for interference measurements, the cordless cellular base station can advantageously use the same receiver for both tasks.